Generally, Multiple-Input Multiple-Output (MIMO) technology will hereinafter be described in detail. In brief, MIMO is an abbreviation for Multiple Input Multiple Output. MIMO technology uses multiple transmit (Tx) antennas and multiple receive (Rx) antennas to improve the efficiency of transmit/receive (Tx/Rx) of data, whereas the conventional art generally uses a single transmit (Tx) antenna and a single receive (Rx) antenna. In other words, MIMO technology allows a transmitting end and a receiving end to use multiple antennas so as to increase capacity or improve performance. If necessary, the MIMO technology may also be called multi-antenna technology.
In order to support MIMO transmission, a precoding matrix that properly distributes transmission information according to a channel condition and the like can be used. The conventional 3GPP system supports a maximum of 4Tx antennas for downlink transmission, and defines a precoding codebook for a maximum of 4Tx antennas.